Making a fractured powder metal connecting rod

ABSTRACT

A method of making a connecting rod for attachment to a bearing journal by separation of parts of the connecting rod, comprising: (a) forging a powder metal sintered preform to provide a one-piece connecting rod having an annular wall defining a crank opening with a center axis and with stress risers for establishing a cracking plane that extends across said crank opening; (b) providing access for a compression coupling across the cracking plane; (c) while at ambient conditions, applying tension substantially uniformly across the cracking plane to propogate fracture from the stress risers along said cracking plane and thereby separate the connecting rod into a cap and body with cracked surrfaces; and (d) remating the cap and body by applying a compression coupling through the access to draw said cap and body together under guidance and with metal yielding pressure to effect substantially an exact rematch of said cracked surfaces. Control of the diametrical clearance between the bolt shanks and the bolt openings, of the bolts used as the compression coupling, promotes guidance needed to achieve such rematch. The cracking is effected in an improved manner by use of continuous pulling apart of the rod in a direction perpendicular to the cracking plane.

This is a division of application Ser. No. 07/194,750, Filed May 17,1988 pending.

BACKGROUND OF THE INVENTION 1. Technical Field

This invention relates to the art of making annular bearing assemblieswith separable journalizing parts that permit reception of a shaft otherthan along an axial direction of the bearing; and, more particularly, itrelates to the art of making split connecting rods for use in automotiveinternal combustion engines.

2. Description of the Prior Art

Automotive connecting rods usually have one end that forms part of anannular bearing assemblY requiring separable cap and body portions topermit insertion of a complex configured crankshaft from a direction notalong the axis of the bearing. A simple pin bearing assembly at theopposite end of the connecting rod usually attaches the rod to a piston;the simplicity of this attachment allows the pin to be received alongthe axis of the bearing.

The advent of more compact engines delivering higher horsepower atincreasingly higher rpm's has placed increased stress on the connectingrod and its bearings. The bipartite rod should act as a unitary piece totransfer dynamic forces with better bearing life. To meet thischallenge, the manufacture of automotive connecting rods has undergoneevolutionary changes.

Connecting rods were originally made by separately casting or forgingattachable cap and body portions. These parts were usuallY made of highcarbon wrought steel and were separately machined at both joining facesand thrust faces; they were then separately with holes to acceptfasteners. A first evolutionary step was to cast or forge the connectingrod as a single steel piece, followed by the drilling of holes to acceptfasteners. The single piece was sawed to obtain cap and body portionswhich were separately rough-machined at the thrust and contactingsurfaces; the two portions were then bolted together forfinish-machining. The separate steps of machining and sawing were notonly cumbersome and expensive, but they also did not ensure a perfectlymatched cap and body under all operating forces. The bolts cannot ensureperfect doweling of the cap and body portions upon bolting togetherbecause of some diametrical clearance existing between the unthreadedbolt shank and the opening through which it extends. The bolts weretorqued to apply compression forces that would prevent relative movementbetween the portions. Unfortunately, under some operating conditions,this inherent fastener clearance may permit slight microshifting of thecap and body portions which may affect bearing life.

As the next step in this evolutionary change, a single-piece connectingrod was split into its mating cap and body portions with an undulatinginterface in the hope of providing nonsliding surfaces where the cap andbody portions are bolted together. If such surfaces were properlyremated, the remate should prevent microshifting and assure moreaccurate operating alignment.

To split the single piece into two, it was initially struck on one sidewith a sharp blow. This met with little success because of theuncontrollability of the cracking plane and possible damage to theconnecting rod. An early attempt at nonimpact splitting was accomplishedby fracturing the big end of the connecting rods with a wedge-expandablemandrel placed in the crank bore opening (see U.S. Pat. No. 2,553,935).All finish machining was conducted on the one-piece connecting rodbefore fracturing. Even though the rod was made of a strong, nonbrittle,high carbon wrought steel, fracturing was carried out at roomtemperature. Brittleness across the cracking plane was achieved bycutting deep radial reductions at the crack plane--by sawing, milling,and drilling, or combination of all three--to significantly reduce thecrackable section. Such connecting rods were intended for light dutyapplications such as small outboard marine engines and lawnmowers.

Another approach to splitting was disclosed in U.S. Pat. No. 3,751,080,which recognized the difficulty of fracturing strong high carbon steelsat room temperature when they were formed in large sizes adequate forautomotive engine applications. An electron beam was moved along adesired splitting plane in an undulating fashion to render a pair ofrippled interfacing surfaces. Again, all machining was accomplishedprior to the splitting. This technique may be undesirable not onlybecause a high energy electron beam can have a deleterious effect uponmaterial performance, but also because it is considered slower and morecostly than previous techniques.

A recent attempt at splitting is disclosed in U.S. Pat. No. 4,569,109,which suggests that the rod can be composed of either cast iron,aluminum, or steel that is made brittle by freezing or heat treatment.Such materials can then be fractured by applying tension across acracking plane while limiting relative movement of the cap and bodyportions to avoid bending or incomplete fracture (the material havingsufficient ductility to provide this risk). Again, this method providesall finish-machining prior to cracking. Disadvantages peculiar to thistechnique are: (a) To avoid freezing or unnecessary heat treatment, castiron or aluminum must be used that does not provide adequate. tensilestrength for a given size; thus, a more massive rod is necessary toachieve higher strengths, which is counterproductive both to fracturingat room temperature and to a better balanced rod. (b) Because allshaping or machining must be carried out prior to cracking, thetechnique suffers from association with wrought materials. (c) Separatemachining must be provided to make the rod sensitive to cracking. (d)Marginally ductile materials cannot always be cleanly cracked. (e)Crack-initiating notches in the crank bore wall provide inadequatesupport for insertion of a bearing member.

There are also certain disadvantages common to all of the prior artsplitting techniques: (a) the bolts, when assembled into both the capand body portions of a split rod, are retained loosely in place untilfinal assembly, subject to being unintentionally unscrewed and misplacedand thereby permitting mix-up of mating parts; (b) the clearance betweeneach bolt shank and bolt opening is not controlled sufficiently toprovide an adequate guide to remating the cap and body portions atidentically their exact separation location; (c) some slight distortionin the roundness of the bore opening in the cap and body portions mayaccompany room temperature splitting by tension and is not compensatedduring reassembly thereby detracting from the accuracy of the finalassembly; and (d) the need to machine locking notches in the internalsurface of the bore opening wall while the cap and body are separated.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for making an improvedconnecting rod that is particularly useful for the demanding conditionsof improved compact automotive engines having higher power. Theinvention, in its several aspects, also overcomes the disadvantageslisted above.

The method of this invention, in a first generic aspect, comprisesmaking a connecting rod for attachment to a bearing journal byseparation of parts of the connecting rod, comprising the steps of: (a)forging a powder metal sintered preform to provide a one-piececonnecting rod having an annular wall defining a crank opening with acenter axis and with integral or subsequently machined stress risers forestablishing a cracking plane that extends across the crank opening; (b)providing access for a compression coupling across the cracking plane;(c) while at ambient conditions, applying tension substantiallyuniformly across the cracking plane to propogate fracture from thestress risers along the cracking plane and thereby separate the rod intoa cap and body with cracked surfaces; and (d) remating the cap and bodyby applying a compression coupling through the access to draw the capand body together under guidance and under yielding pressure to effectsubstantially an exact rematch of the cracked surfaces.

The stress risers may be crack-initiating crevices, preferably providedby preformed indentations in the annular wall at opposite sides of thecrank opening and extending substantially parallel to the center axis.Alternatively, the stress risers are provided by at least two machinedgrooves at opposite sides of the inner bore wall again extendingsubstantially parallel to the center axis. Cracking can be carried outby use of an expandable mandrel in the crank bore opening to internallycreate tension or by use of a device to continuouslY pull the rod apartexternally creating tension. Guidance in step (d) is preferably carriedout by controlling the diametrical tolerance of the compression couplingand its access opening to 0.006-0.014 inches; the draw of such couplingis carried out with a force substantially up to the yield point of themetal comprising the compression coupling.

Preferably, the forged powder metal one-piece rod has a sharpy V-notchtest value of 10-15 ft/lbs at 70° F. and a hardness of 30-40 R_(c). Morepreferably, the powder metal rod has tensile test properties of 130-140ksi, a yield strength of 100-115 ksi, elongation of 10-15%, and areduction of area at the yield point of 30-40%.

As a second method aspect of this invention, the method is more specificwith respect to how the stress risers are created and how cracking bytension is carried out. Specifically, the stress risers (forestablishing a cracking plane) are defined independently of the forgingstep, and the fracturing step is carried out to continuously pull apartthe single-piece connecting rod in a direction perpendicular to thecracking plane and with no limit on relative separation movement alongthe direction of pulling. With reference to the direction of pulling,the cap portion may be held stationary and only the body portion pulledalong such direction; alternatively, the body portion may be heldstationary and the cap portion pulled away. More preferably, the capportion is made substantially square to the direction of pulling bystops limiting skewing deviation and the body portion is maintainedsquare to the pulling direction by securing the pin end of the bodyportion to a slidable support and sliding such support along a trackparallel to the pulling direction. Advantageously, for automotiveconnecting rods, the ratio of the mass (in grams) of the rod to thecross-sectional cracking plane area (in square inches) of the rod is inthe range of about 400 to 800.

A third method aspect of this invention is more specific as to how thesplit portions are guided during remating and employs a unique mode formaking lock notches in the internal crank bore wall. Specifically, amock journal is used to guide the application of compression forces, andfull radius bearing locking notches are provided at each diametricalside of the annular wall extending across the cracking plane with eachnotch having a curvilinear radial extremity throughout its length.

The apparatus aspect of this invention comprises: (a) a member forholding one internal side of a bearing generally stationary, the oneside being on one side of an intended cracking plane; and (b) means forcontinuously pulling a second internal side of the bearing away from thefirst side in a direction perpendicular to the cracking plane while thesecond internal side is held substantially square to the direction ofpulling.

This invention also considers the resulting bipartite connecting rod aninventive improvement, characterized by: (a) a body and cap, eachconsisting of densified powder metal fractured from a unitary hot forgedpowder metal preform of substantially net shape, the body and cap beingremated at substantially the exact rematch of the fracture surfaces andsecured in such rematched position by threaded compression fastenersextending across the cracking plane; (b) clean cracked surfacesextending along the cracking plane with little or no raised lips oredges deviating therefrom; (c) improved bore tolerances to 0.0004 inchesa mass/cross-sectional cracking area ratio of 400-800.

SUMMARY OF THE DRAWINGS

FIG. 1 a perspective view of a forged one-piece powder metal connectingrod which is to be fractured according invention;

FIGS. 2 (A-C) are schematic flow diagrams of the different methodaspects of this invention;

FIG. 3 an elevational view, partly in section, of an apparatus useful incarrying out the method aspect of 2A;

FIG. 4 a side elevational view taken along line 4--4 of FIG. 3;

FIG. 5 is a fragmentary perspective view of a portion of the cap of theconnecting rod showing the fractured surface thereon;

FIG. 5a is an enlarged side view of the fractured surfaces of both thecap and body;

FIG. 6 is an enlarged exploded perspective view of a portion of thelarge end of the connecting rod and a fastener used to apply compressionforces between the cap and body;

FIG. 6a is an enlarged sectional view of a portion of the fastenerinserted into the fastener opening of the rod in FIG. 6, andadditionally shows an interference-fit ferrule position;

FIGS. 7 and 8 are, respectively, side elevational and perspective viewsof a device used to guide and draw the split cap and body portionstogether about a mock bearing journal;

FIG. 9 is a side elevational view, partly broken away, to illustratelock notches in the crank bore opening;

FIG. 9a an enlarged sectional view taken substantially along line 9a--9aof FIG. 9;

FIG. 10 is a front elevational view connecting rod processed inaccordance with this invention and shown assembled with a crank armbearing therein;

FIG. 11 a schematic perspective view of a pulling apparatus useful incarrying out the method aspect 2B;

FIG. 12 is a sectional view taken along line 12--12 of FIG. 11;

FIG. 13 a perspective view of the connecting rods, made by thisinvention, assembled in an automotive engine.

DETAILED DESCRIPTION AND BEST MODE Powder Metal One Piece Rod

Common to all the method aspects of this invention is the use of apowder metal sintered preform which is hot formed to near net shape of aconnecting rod. By the mechanism of controlled sintering of alloyedmetal powder, followed by controlled cooling, a connecting rod can beformed which possesses desirable notch sensitivity so that it can besplit with a desirable fracture pattern that ensures improved remating.For powder steel connecting rods, a sharpy V-notch value of 5-10 ft/lbsat 70° F., along with a hardness of 15-27 R_(c) (75-88 RG) is desirable.Such notch sensitive steel material can be attained even though theconnecting rod will have tensile test physical characteristics which mayinclude an ultimate tensile strength of 120-140 ksi, a yield strength of80-100 ksi, an elongation of 10-15% , and a reduction of area at yieldof 20-30%.

For powder aluminum alloy connecting rods, the sharpy V-notch value willbe slightly lower than that of powdered steel at 70° F.; such notchsensitive aluminum alloy material can be attained even though the otherphysical properties are enhanced by fiber reinforcement in other regionsof the rod.

To this end, a steel powder metal net shape connecting rod is formed bythe following steps: (a) admixing (percent by weight of the admixture) aferrous powder with 0.4-0.8% graphite and 1.5-2.5% copper, the admixtureshould contain no more than 1.5% impurities and should have an averageparticle size of about -200 mesh; (b) the admixture is compacted undersufficient force to define a preform having a density and configurationto facilitate handling and subsequent hot forming into a desired shape;(c) the preform is sintered in a low oxygen potential atmosphere (as lowas 80 ppm) at a temperature of at least 20°-50° F. (preferably 2350° F.)(1287.8° C.) for about 10-15 minutes; the low oxygen potentialatmosphere may be obtained by using a dry hydrogen atmosphere,dissociated ammonia or nitrogen/hydrogen mixtures dried by usingmolecular sieves; (d) hot forming the sintered preform at a temperatureof about 1800°-1900° F. under pressure of 50-100 tons per square inch todefine a forged shape having a density in excess of 99% (about 7.82grams/cm³ minimum); and (e) controllably cooling the hot formed preformto form a ferrite-pearlite microstructure; this can be carried out byallowing it to cool in the furnace to a temperature of 1700° F., andthereafter in aqueous solution or air to ensure avoidance of carbidesbut also avoidance of too soft a material.

The ferrous powder may be a water-atomized soft iron powder; theadmixture may contain not only copper and graphite, but also manganesesulfide, and a wax lubricant added to facilitate ejection of thecompressed powder admixture from the die. The final composition of thesintered metal may be about 0.5% C, 2% Cu, 0.1% S, 0.% Mn, and thebalance Fe.

Alloying ingredients may be employed in the iron powder admixture. Someof such alloying variations are more fully described in U.S. Pat. No.4,069,044, assigned to the assignee of this invention, and thedisclosure of which is incorporated herein by reference. But suchalloying agents must be moderated to ensure a powder metal of a reducedsharpy V-notch value to achieve brittle fracture.

The density of the preform is preferably 6.5 grams/cm³ which is about82% of theoretical density of steel, the remaining 18% consisting of waxlubricant and porosity. The wax is removed by vaporization during theearly heating of step (c) which is carried out in a rotary hearthfurnace. In the forging step, the sintered preform is preferablydensified with one blow of the press having a prelubricated andpreheated die cavity. If needed, the hot formed rods are deburred toremove flask and double disk ground to meet dimensional thicknessspecifications.

Generic Method Mode

As shown in FIG. 2A, a first or generic method mode broadly comprises:(a) forging a powder metal connecting rod with stress risers forestablishing a cracking plane; (b) providing access for a compressioncoupling across the cracking plane; (c) cracking the connecting rod bytension along the cracking plane; and (d) remating the split portionsunder guidance and under yielding pressure. Implied steps which are notpart of the essential steps of mode A, but which are used to make therod, comprise: (e) finish-machining of the mated rod, and (f) assemblyof the finish-machined rod onto a shaft with a bearing assembly.

To carry out step (a), the powder metallurgy techniques above-describedare utilized. As shown in FIG. 1, the hot formed preform provides aconnecting rod 10 having a big end 11 adapted for holding a crank armand bearing assembly in a crank opening 12 defined by annularcylindrical wall 13 having a center axis 14. The rod 10 has shoulders 15and 16 respectively at opposite sides of the rod (taken with respect toa longitudinal axis 17 of the rod). At the small end 18, a cylindricalbearing wall 19 is provided to receive a pin bearing along a center axis20. The shoulders 15 and 16 are designed with sufficient mass toaccommodate fastener openings 21 in each side and have an axis orientedto be parallel with the axis 17.

Stress risers 22 and 23 are incorporated into the preform to establish acracking plane 24. The stress risers permit initiation of fracture andpropogation of the crack across the weakest plane of the materialperpendicular to the direction of tension. Such stress risers may becrevices on the wall 13 which extend parallel to the bore axis 14 ofbore opening 12 and extend throughout the thickness 25 of the rod(longitudinal extent of opening 12). Such crevices may preferably beincorporated as an open groove or fold in the hot forming stage whenmaking the near net shape preform, and such hot formed groove or foldmay be further machined or broached to have sharply converging wallswith an included angle of 70°-100° . Alternatively, such stress risersmay be initially formed by machining or broaching sharp grooves in theunmachined or rough-machined wall 13; such crevices or grooves shoulddesirably have a sharp convergence of walls and a depth of about 0.030inches so that upon application of tension to the inside of the boreopening and across the cracking plane, the crack will propogate sidewaysalong such plane 24.

For purposes of step (b), the openings 21 constitute an access forthreaded fasteners which together with the rod serve as a compressioncoupling (see FIGS. 1 and 6). These openings are drilled into theshoulders 15 and 16 across the cracking plane 24. The portion of eachopening 21, that resides on the side of plane 24 opposite from where thedrill enters the rod, is tapped to provide threads 26. The other portionof the opening 21 is unthreaded and preferably is designed to maintain adiametrical clearance of about 0.001 inch with the bolt shank thatextends therethrough.

Prior to the cracking step, the one-piece rod is not only drilled andtapped to provide the access, but may also have (i) crank opening wall13 and pin opening wall 19 rough-machined, (ii) thrust surfaces 27 (onopposite sides of the rod and about the crank opening 12) ground toprovide perpendicular thrust faces with respect to axis 14, (iii) thrustsurfaces 28 (on opposite sides about pin wall 19) ground to providethrust faces perpendicular to axis 20, and (iv) bolt head seats 29machined or spot faced to provide accurate surfaces perpendicular to theaxes of the cylindrical openings 21 (which are parallel to axis 17) andparallel to the axis 14.

Step (c) is carried out to create tension across the cracking plane byapplying an expanding pressure to the inside of the crank opening wall13. To this end, and as shown in FIGS. 3 and 4, the one-piece connectingrod is loosely supported in a vertical position against a web 30 havingdowels 31 and 32 projecting outwardly to seat the small end 18 of therod, and blocks 35 and 36 extend outwardly to loosely cradle the sidesof the big end 11 while blocks 33 and 34 extend outwardly to looselycradle the sides of the small end 18.

An expandable mandrel 37, carried by a slide apparatus 38, is insertedinto the crank opening 12 of the supported connecting rod. The mandrelhas two oppositely acting members 40 and 41, each respectively providedwith a hemisperical-like surface 42, 43 which can engage only thatportion of the wall 13 which lies on one side of the cracking plane 24.The wedge 39 is interposed between the members 40 and 41, and whenpulled to the right (by an actuating cylinder 44 operating through ashaft 45 attached to an extension 46 of the wedge 39 via a coupling 47),the wedge 39 will cam mating surfaces 48 and 49 of such members radiallyoutwardly and perpendicular to the cracking plane 24. The members areloosely restrained about the wedge 39 by collar 50 which does notinhibit expansion for cracking.

The slide apparatus 38 supports the wedge connection of extension 46,shaft 45 and coupling 47 in a slide carriage 51; the carriage in turn isslidably supported on a pair of rails 52. An actuator 53 is employed tomove the slide apparatus to the left via a rod 53a for positioning ofthe mandrel 37 in the rod opening 12. When the insertion is completed,the actuator 44 can then draw the wedge 39 to the right to burst the rodapart into a cap 52 and body 53.

If the mandrel is accurately mated to the connecting rod and the cammingangle 55 is sufficiently gradual (in the range of 10°-25° taken withreference to the cracking plane 24), the connecting rod will havefractured surfaces 56 and 57 at each side of the rod (one such surfaceis shown in FIG. 5). Such surfaces will be characterized by uniqueraggedness (equivalent to unique fingerprints) that results frommicroyielding deformations or peaks 58 as shown in FIG. 5a.

Step (d) involves remating the split portions (cap 52 and body 53) withan exact identity of the unique fracture raggedness (or fingerprints) asshown in FIG. 6a. Two things are employed to accomplish this: thethreaded fasteners 60 are inserted into the fastener openings 21 to drawthe cap 52 and body 53 together (see FIG. 6). Micromisalignment of thecap and body is avoided by control of the diametrical clearance betweenthe shank 60a of each bolt and the unthreaded portion of opening 21,such as within 0.006-0.014 inches.

It is advantageous that the fastener 60 (i.e., steel alloy (Class 8)bolts) be stressed (by turning with high torque against the threads 54of the opening) up to the yield point of the steel material of the boltshank. The yield point of peaks 58 of the ragged fracture will be lowerthan the yield point of the alloy steel bolt 60. It is important thatmicroalignment of the cap and body be provided because any mating peaksand valleys not realigned will undergo some ductile deformation whilethe cap and body are compressed together. This facilitates mating butreduces interlocking effect between the peaks of the mating surfaces.Any micromisalignment can further be obviated by use of a mock journalguide 67 (see FIGS. 7 and 8), and any out-of-roundness of the wall 13 ofthe cap and body will also be restored during such remating step. Themock journal is inserted into the crank opening 12 with the diametricalclearance between the mock journal outer surface 67a and internal crankopening wall being about 0.0006 inches or less. The mock journal shouldbe the substantial equivalent of the combination of the bearing sleeve64 and crank arm 65 which is to be inserted and received by theconnecting rod big end. Advantageously, such mock journal may besupported on an upright stand 62 and coupled thereto by fasteners 63.The support orients the mock journal to be square to the axes 14 and 17of the connecting rod.

This completes the essential steps of this method mode, but theadditional step (e) of finish-machining of the mated rod portions may becarried out. Such machining is accomplished (i) at the crank boreopening 12 to remove material to a depth of about 0.017 inches, whichmay be about 60-75% of the depth of any crevice in the wall 13; (ii) atthe thrust surfaces 27 by a light grinding to remove material only to adepth of about 0.005 inches; and (iii) at locations for milling oflocking notches as will be described in connection with method mode 2C.

A conventional step (f) would involve unloosening the bolts to a degreepermitting separation of the cap and body necessary to allow theinsertion of the half shell bearings into each part of the connectingrod and the assembly of such body and cap about the right crank arm ofan automotive crankshaft 92. To facilitate retention of the bolts 60 tothe cap of the connecting rod, interference-fitting bands 69 or ferrulesmay be slipped onto the shank of each bolt prior to formation of threadson each bolt; when the bolt is fully threaded into opening 21, the bolthead 60b will force the band or ferrule into place as an interferencefit (as shown in FIG. 6a). Thus, even when bolts 60 are fully unthreadedfrom body 53, the band 69 will not permit the threaded portion of eachfastener to pass through such band and thereby will be retained to cap52. Thus, loss of bolts during attachment to the crankshaft is avoidedand automation of such assemblY is facilitated by such bolt and capassembly.

Method Mode 2B

This mode additionally provides for definition of stress risersindependently of step (a). This may be accomplished by machining orbroaching sharp grooves into the rough-machined surface of crank boreopening 12, such grooves extending parallel to the axis 14 andpreferably to a depth of about 0.030 inches. Again, the converging ofsuch grooves should be sharp and have an included angle therebetween of70°-100° . The machining for this substep (a-1) should be either in therough-machined bore surface or in the virgin wall itself prior torough-machining.

This method mode also distinguishes from the generic mode by aparticular and improved manner of applying tension in step (c). Thismode continuously pulls apart the connecting rod in a directionperpendicular to the cracking plane with no limit on relative separationmovement along the direction of pulling. This may be carried out by theapparatus as shown in FIGS. 11 and 12. A pair of plates 70 and 71 arerigidly, but removably, provided to secure a hemisperical-like block 72therebetween by use of a plurality of dowels 73 extending through theplates and block. Slide plates 74,75 similarly secure ahemisperical-like block 76 (in opposed position to block 72) by dowels77. However, a pin 78 (having a diameter commensurate with the internaldiameter of the pin opening wall 19 of the small end 18 of theconnecting rod) is rigidly held by the plates 74,75 and is received inthe small end of the rod to permit the slide plates 74,75 to pull thesmall end of the connecting rod. The pulling force is provided by anactuator 79 and is oriented to provide a pulling force that isperpendicular to the cracking plane 24 as aided by tracks 85 on a bed 86guiding the sliding motion of plates 74,75 by complementary rails 87.

Tension created by tracked continuous separation movement provides anability to crack marginally ductile connecting rods without evidence ofdistortion. However, it is desirable that the hardness of the powdermetal rod be maintained in the high end of the preferred range toenhance consistency of cracking.

The roundness of the crank bore opening will quite unexpectedly bebetter retained with little or no evidence of distortion. Anyout-of-roundness will be removed by finish-machining, but with increasedefficiency due to accuracy of the opening. The cracked plane will bedevoid of raised lips or ridges which can be a product liability problemif such lips are knocked loose in operation and enter the lubricationsystem.

Method Mode 2C

This mode provides for mock journal guiding during remating and fullradius bearing locking notches. Step (d) remates the split bearingportions (cap 52 and body 53) guided bY a mock journal 61, usingcompression forces (applied by threaded fasteners 60 in threadedopenings 21). Step (e) is divided into two stages, the first of which isto machine the crank bore wall 13 to finish dimensions which removessome portion of the stress risers 22 and 23. As shown in FIGS. 9, 9a,and 10, the second stage of (e) is to provide one or more lockingnotches 80 in the finish-machined wall 13, which notches extend acrossthe cracking plane 24 and have a curved radial extremitY 81 throughout.The notches 80 may be created by milling grooves with a rotarY cuttingtool that has a cutting radius substantially less than the radius of thewall 13. The grooves may have a rectangular cross-section defined by awidth 82 of about 0.25 inches and a groove bottom (radial extremity 81)that extends a distance 99 (0.5-1 inches) to either side of the crackingplane 24. These notches provide a sharp shouldered space into whichtangs 83 and 84 of half round or shell bearing sleeves 64 may radiallyextend thereinto. Such half shell bearing sleeves are locked againstaxial displacement, taken with respect to axis 14, provided the tangsfit snugly into the width of such grooves.

Step (f) comprises the assembly of the cracked connecting rod onto ashaft 65 (i.e., crank arm of an automotive crankshaft 92) with the halfshells of the bearing sleeve 64 interposed therebetween. Such bearingsleeve can be comprised of laminated bronze powder metal coated with ababitt-type material. The half shells are shrunk-fit to the crank boreopening wall 13 and aligned so that the shells separate with the cap andbody respectively during assembly onto the shaft 65.

The resulting bipartite connecting rod is characterized by: (a) a bodyand cap, each consisting of densified powder metal fractured from aunitary hot forged powder metal preform of substantially net shape, thebody and cap being remated at substantially the exact rematch of thefracture surfaces and secured in such rematched position by threadedcompression fasteners extending across the cracking plane; (b) clean,cracked surfaces extending along the cracking plane with little or noraised lips or edges deviating therefrom; (c) improved bore tolerancesto 0.0004 inches or less; and (d) a mass/cross-sectional cracking arearatio of 400-800.

The apparatus aspect of this invention comprises: (a) a member forholding one internal side of a bearing generally stationary, the oneside being on one side of an intended cracking plane; and (b) means forcontinuously pulling a second internal side of the bearing away from thefirst side in a direction perpendicular to the cracking plane while thesecond internal side is held substantially square to the direction ofpulling.

While particular embodiments of the invention have been illustrated anddescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from theinvention, and it is intended to cover in the appended claims all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

We claim:
 1. A bipartite connecting rod for attachment to the crank armof an automotive crankshaft for a reciprocating engine, characterizedby:(a) a body and cap, each consisting of densified powder metalfractured from a unitary hot forged powder metal preform of substantialnet shape, said body and cap being remated at substantially the exactrematch of said fractured surfaces and secured in such rematchedposition by threaded compression fasteners extending across the crackingplane; (b) the rod having a ratio of its mass to surface area in therange of 200-800; (c) a crank bore deviation within the tolerance rangeof plus or minus 0.006-0.014 inches; and (d) generally planar fracturedplane devoid of lips or edges projecting from said plane.
 2. Theconnecting rod as in claim 1, which is further characterized by ease ofmilling due to improved short chip formation, the reduction of energynecessary for machining, and the reduction of abrasion to the cuttingtool.